1. Field of the Invention
The present invention relates to a permanent magnet type rotary electric machine. In particular, the present invention relates to a motor structure for improving saliency, a motor structure for reducing angle error in sensorless drive, and an electric power steering apparatus to which a permanent magnet type rotary electric machine can be applied.
2. Description of the Related Art
In order to drive a synchronous motor such as a permanent magnet type motor, it is necessary to perform position sensing using a rotary sensor or the like. On the other hand, there are developed sensorless drive methods for driving the motor without using a rotary sensor so as to reduce cost of the motor or size of the same. One of the methods is the high-frequency voltage injection method in which a high frequency voltage is applied to an armature winding of the motor in addition to a voltage for generating a torque, and position sensing of a rotor is performed by utilizing a difference between d-axis current and q-axis current due to dependency of motor impedance on a rotor position (i.e., saliency). However, this method requires the saliency. An interior permanent magnet motor has the saliency, and thus can be used for the sensorless drive (see, for example, Japanese Patent Application Laid-Open No. 2004-056871 and Japanese Patent Application Laid-Open No. 2004-350380).
However, the conventional technique has the following problems.
The conventional technique has a problem in that the rotor has a larger cogging torque because of an interior permanent magnet motor compared with a surface permanent magnet motor. In addition, there is another problem that because the angle error of the position sensing in the sensorless drive varies in accordance with motor load current, the torque may be lowered, or “step out” or the like may occur. In addition, the saliency is small, and hence it is necessary to increase the high frequency current for the position sensing in the sensorless drive, which causes a problem of large noise and a further problem that the saliency varies in accordance with the load current.